The invention relates to a device of irradiation measurement comprising a phantom made of a material equivalent to the system soft tissues showing a cavity for housing a radiation detector.
A directive of the Euratom Council determines the basic standards relative to the sanitary protection of the population and the workers against the dangers resulting from ionizing radiations. This directive paints out the ideas of effective dose and dose index, the effective dose being the sum of dose positional mean equivalents in the different organs or tissues, and the dose index being the equivalent of the maximum dose in the middle volume of a 30 cm diameter sphere made of a material equivalent to soft tissues with a volume mass of 1 g per cm.sup.3. An index of deep dose equivalent and an index of surface dose equivalent are distinguished and if the new dose limits to be abided for the workers or the public are established in terms of effective dose, these dose limits are considered as abided if the index of deep dose equivalent does not exceed the stated limit for the effective dose and if the index of surface dose equivalent does not exceed the stated dose limit for the skin.
The practical devices actually available do not meet the aims stated by the above-mentioned requirements. An object of the present invention is to permit the realization of a measuring device allowing the practitioner of the radioprotection close to the nuclear or radiation generating plants to determine rapidly and conveniently under all circumstances the variation of the absorbed dose rate versus depth in a phantom simplified and made of a material equivalent to the system soft tissues concerning the interactions of electrons, photons and neutrons with the matter.
According to the present invention the measuring device is characterized in that the phantom includes a first rotary right cylinder having a cylindrical housing with an axis parallel and eccentric relative to the axis of the first cylinder, and a second revolution cylinder mounted in rotation with weak play in the cylindrical housing, the cavity being eccentrically accommodated in the second cylinder, so that the combined rotations of the first and second cylinders make the depth of the detector vary in the phantom according to the diameter of a circle of the first cylinder.
The theoretical use of a 30 cm diameter sphere for these measurements shows the disadvantages to be of a not very convenient realization and working, and according to the present invention, the sphere is advantageously replaced by a right cylinder 30 cm in diameter and 30 cm in height allowing the required measurements with a largely sufficient approximation. The second cylinder of same height and with a diameter close to 15 cm is eccentrically disposed within the first cylinder. The two cylinders have parallel axes and are actuated with rotation motions in opposite direction, the rotation angle of the first cylinder being half of the rotation angle of the second cylinder. It will appear from the following description that the device with eccentric cylinders according to the invention permits to displace the cavity and the detector located in the cavity according to a diameter of the first cylinder, this displacement being able to be at constant speed if one takes care in modulating the rotation speeds of the cylinders. The device can then provide the direct recording of the variation of the absorbed dose versus depth when it is a unidirectional or isotropic irradiation. The rotation motion of the second cylinder is advantageously derived from the one of the first cylinder, for example by a mechanical connection with gearing, every other drive device being of course usable. The rotation of the first cylinder can be automatically realized by a motor controlled by a logic system with wired memory and with a selective time base allowing different transit times with uniform speed of the cavity. The simultaneous recording of the dose rate in the detecting cavity permits a direct indication and a recording of the variation of this dose rate versus depth in the phantom.